The present invention relates to a press as well as to a method for operating a press. The press comprises a ram that can be moved via a press drive. The press drive comprises a motor as well as a press transmission connecting the motor with the ram.
Depending on its design, the press transmission comprises one, or also more, sliding bearings. On each sliding bearing, a first transmission element is supported so as to slide on a second transmission element. A servo press comprising sliding bearings has been known, for example, from publication DE 10 2008 028 652 B3. It describes an eccentric mechanism, wherein the eccentric rod is rotatably supported on the eccentric via a sliding bearing. The other axial end of the eccentric rod is connected with the ram via another sliding bearing.
Such sliding bearings require lubrication. In order to avoid any undesirably high wear, a lubricant film having a minimum thickness must be interposed between the two transmission elements that are supported against each other via the sliding bearing. A hydrodynamic lubrication is achieved as long as the two transmission element move or rotate at a sufficiently high relative speed relative to each other. In such a press operating state, a lubricant film is achieved, said lubricant film having sufficient thickness to keep the wear on the sliding bearing minimal and to avoid damage on the sliding bearing. This requires a lubricant film thickness that is greater than the largest particles of debris that are present in the lubricant.
Presses and, in particular servo presses, can be operated with variable position and/or force controls of the ram. Until now, sections in the motion profile of the ram, in which said ram moves either very slowly and/or the application of a ram force is temporarily stopped altogether, have been avoided. For example, the ram may stop in the region of its lower reversal point in order for processing the reshaped workpiece, for example by welding. In the case of molding presses, it may also be necessary to maintain the ram force for a certain period of time in the lower reversal point of the ram. Furthermore, the press drive may also initiate a pendulum movement of a drive shaft so that the two transmission elements of the sliding bearing only move in pendulum fashion or oscillate relative to each other within a certain angular range and no longer rotate relative to each other.
Considering these aforementioned examples of motion and/or force control of the ram, it can be difficult to generate a sufficiently thick lubricant film on the sliding bearing. Therefore, there is the risk that particles of debris contained in the lubricant cause increased wear and ultimately lead to a prematurely defective sliding bearing.
Consequently, the object of the invention may be viewed to be the provision of a press and a method, respectively, for operating a press wherein the lubrication of the sliding bearing of the transmission is improved.